Abstract: Retroreflective material including a medium containing a plurality of diffraction elements comprising light interference fringe patterns is formed by pressing a first medium having on a surface thereof a plurality of light interference fringe patterns defining an array of phase modulated fresnel zone plates onto a surface of a copy medium having a thickness equal to a given focal length associated with the fresnel zone plates and a reflective material is coated on the opposite surface of the copy medium. In making other embodiments a plurality of fringe patterns is recorded throughout the volume of a photosensitive medium.

Abstract: An improvement in the known method of tagging individual units of production of a substance with microparticles for retrospective identification is disclosed. The improvement comprises the use of microparticles which are encoded with an orderly sequence of visually distinguishable colored segments. Decoding of the microparticles can be accomplished with a microscope or other magnifying device.

Abstract: Retroreflective material including a medium containing a plurality of diffraction elements comprising light interference fringe patterns for enabling provision of a retroreflective effect upon light incident thereon. In making one embodiment a plurality of light interference fringe patterns defining an array of phase modulated fresnel zone plates is replicated by embossing at one surface of a copy medium having a thickness equal to a given focal length associated with the fresnel zone plates and a reflective material is coated on the opposite surface of the copy medium. In making other embodiments a plurality of fringe patterns is recorded throughout the volume of a photosensitive medium.